Surface treatment is conducted on a coated cutting insert in order to remove coated surface defects and improve surface roughness, thereby reducing cutting resistance and preventing the fusion of the coated surface with a work piece. Conventionally, cutting inserts coated with CVD or PVD have been surface-treated by methods of blasting, polishing, brushing and the like. For such surface treatments, the coated cutting insert is fixed within a fixture, as shown in FIG. 1.
The conventional fixture (2) seizes only a lower part of a cutting insert (4), while an upper part of the cutting insert (4) partially protrudes from an upper surface of the fixture (2). By doing so, the insert (4) is surface-treated by brushing and the like. For example, a rotating brush can be used for surface treatment. During surface treatment by a rotating brush, the cutting edge R (6) is subject to a larger load compared to that of a rake surface (8) (the load mainly includes a pressure force of the brush and partially includes an impact or frictional force from the brush). This is because the rotating brush, which has a relatively large impact with rotating, hits the protruding cutting edge R (6) first and brushes the cutting edge R (6) with a relatively large load, wherein the brush then proceeds to brush the rake surface (8) with a relatively reduced impact and load. For this reason, the brushing load of the rotating brush is concentrated on the cutting edge R (6) rather than on the rake surface (8). Accordingly, the honed amount (H) (indicating an amount of wearing at the cutting edge R, which is usually measured as a straight distance from a starting point to an end point of the cutting edge R, along the surface perpendicular to a lower surface of the insert) or the radius of curvature of the cutting edge R increases after the surface treatment (rather than before such a treatment), even if the surface treatment period, brushing pressure, rotation speed and the like are controlled. In particular, the portion adjacent to the rake surface (8) of the cutting edge (R) is more severely worn out during the surface treatment compared to the portion adjacent to the flank surface (10). As a result, the curvature radius adjacent to the rake surface (8) of the cutting edge (R) becomes asymmetrically larger than that of the portion adjacent to the flank surface (10).
As the curvature radius of the cutting edge R becomes larger during the surface treatment, there is a problem in that the cutting performance of the cutting insert is degraded and precise machining cannot be performed. This deteriorates the surface roughness of a work piece. In particular, if the curvature radius of the cutting edge R, which should be kept as small as possible, becomes larger in a PVD-coated cutting insert, then the cutting performance of the PVD-coated cutting insert is remarkably degraded.
Further, in the conventional fixture structure, since the insert (4) is disposed protruding from the fixture (2), the load for surface treatment is concentrated on the insert (4) rather than the fixture (2). If the load (e.g., from a rotating brush) is concentrated on the rake surface of the insert, which has chip breakers (not shown) of a highly curved shape, then the protruded portion on the rake surface is subject to a larger load than the recessed portion on the rake surface. Accordingly, the protruding portion is severely worn out relative to the recessed portion and the coated surface of the rake surface is non-uniformly treated. As such, in the conventional fixture structure wherein the insert is fixed protruding from the fixture, it is difficult to achieve a surface treatment that is uniformly conducted on the rake surface having chip breakers of a curved shape. A coated rake surface having chip breakers contacts and bends cutting chips generated during the cutting process, thereby controlling the discharge of cutting chips. The non-uniformly treated and coated rake surface obstructs the smooth discharge of cutting chips, thereby resulting in the degradation of cutting performance.
As such, according to the conventional surface treatment method, the curvature radius of the cutting edge R becomes larger and the rake surface is not uniformly treated. Thus, the cutting performance of the coated cutting insert is degraded and the life of the coated cutting insert is reduced.
An object of the present invention is to provide a new surface treatment method, which can perform coated surface treatment on a cutting edge R and a rake surface, while maintaining a radius of curvature of the cutting edge R and uniformly surface-treating the rake surface.